1. The Field of the Invention
The present invention relates generally to systems and methods for decreasing the overall polarization dependent loss in a series of cascaded optical components.
2. The Relevant Technology
Polarization can be defined as the property of an electromagnetic wave that describes the orientation, i.e., time-varying direction and amplitude, of the electric field vector of the wave. States of polarization are described in terms of the figures traced as a function of time by the projection of the extremity of a representation of the electric vector onto a fixed plane in space, which plane is perpendicular to the direction of propagation. In general, the figure, i.e., polarization, is elliptical and is traced in a clockwise or counterclockwise sense, as viewed against the direction of propagation. If the major and minor axes of the ellipse are equal, the polarization is said to be circular. If the minor axis of the ellipse is zero, the polarization is said to be linear. Rotation of the electric vector in a clockwise sense is designated right-hand polarization, and rotation in a counterclockwise sense is designated left-hand polarization.
The polarization of an optical signal plays an important role in the propagation of optical signals in an optical data network. For instance, the amount of an optical signal, such as light, reflected at the boundary between two different materials, depends on the polarization of the incident wave. The amount of an optical signal absorbed by certain materials is polarization dependent. The refractive indices of anisotropic materials are polarization dependent. Some optically active materials have the ability to rotate the polarization of an input beam of light. In all of the above cases, polarization dependent losses can occur.
A definition of polarization-dependent loss (PDL) is the maximum change in insertion loss due to changes in the polarization state of a signal. It may be quantitatively expressed as:
                              PDL          db                =                  10          ⁢                                    log              10                        ⁡                          (                                                I                  max                                                  I                  min                                            )                                                          (        1        )            where I is defined as the intensity of light. The value for I is equal to E2, where E is a value of the electrical field vector of an optical component for various polarization states. Imax is then equal to Emax2, and Imin is equal to Emin2. PDL is expressed in decibels.
Polarization-dependent loss is a measurable characteristic that can be used to evaluate the insensitivity of an optical component to changes in the polarization state of a light signal passing through the component. A feature of a component in an optical fiber communications network is the component's insensitivity of response to changes in the polarization state of an input light signal.
For any given component in a system, the polarization direction of the output light can be arbitrary, depending on the polarization of the light input into the component. It is entirely possible for light input at one arbitrary polarization direction to result in a minimum insertion loss, while light input at another arbitrary polarization direction results in a maximum insertion loss for a particular component. The difference between the maximum and minimum insertion loss is defined as PDL. A large PDL often results in an increased bit error rate of the optical communication system, which is a serious system impairment.
As components are added (cascaded) in a system, the PDL is cumulative. Since the output polarization direction for a given component in a chain can be arbitrary, the PDL can either cancel itself out, or accumulate additively. A large PDL can adversely affect the optical transmission properties for a given set of components. Thus, PDL is an important characteristic for optical components used in, for example, telecommunications networks. Reducing PDL can be a priority in optical systems.